1. Field of the Invention
The present invention pertains, in general, to manufacturing methods of printed circuit boards, and more specifically, to a method of manufacturing a printed circuit board maximizing efficiency of use of its surface area, and having a fine pattern of its circuit and high integration of the circuit, due to requiring no etching process.
2. Description of the Prior Art
Generally, in conventional fabrication methods of a printed circuit board (PCB), an etching process of a copper foil on a polymer film negatively affects high integration of electronic parts, attributable to fundamental limitations of the etching process. In this regard, U.S. Pat. No. 5,077,084, using the copper foil (photoresist (PR) pattern/copper foil/polymer), discloses a technique of bonding a polymer to the copper foil by casting. However, this technique is disadvantageous in terms of formation of oxides on the copper foil or low bondability of the copper foil. In addition, an etching process (in particular, wet etching) for use in the formation of patterns results in a nonuniform thickness of the cross-section of each line in the copper pattern. Thus line intervals in the copper pattern cannot be decreased. The above technique is unsuitable for application in highly integrated circuits requiring more output terminals and narrower line width.
In U.S. Pat. No. 6,296,949, there is disclosed a metallic layer for increasing bonding strength between a polymer and a conductive copper line, formed by the process of vacuum-metallization, electroless deposition or electrodeposition (copper electroplating and PR pattern/copper electroless plating/polymer) in the presence of palladium catalyst. In the case of electroless or electro deposition process, since a copper electroless plated film is much faster in etching rate than a copper electroplated film, problems such as undercut (phenomenon wherein the electroless plated film under electroplated film is etched too much) may be caused. Also, although the polymer surface should be subjected to surface modification upon use of an adhesive layer formed of the polymer, the polymer material having excellent etching resistance, for example, polyimide, is difficult to perform surface modification on. So, the above process is disadvantageous in terms of limitation of usable polymer materials. Meanwhile, in the case of the vacuum-metallization process, a production cost is increased due to vacuum process, and this process is not closely connected with other wet processes and thus cannot be efficiently applied to a continuous process including a roll-to-roll process.
On the other hand, blind via holes have been typically formed in an insulating layer on the substrate by laser or exposure, and a conductor circuit layer is formed on the surface of the insulating layer and in the via hole by means of electroless copper plating. Thusly formed conductor circuit layer is connected to the lower conductor circuit layer, thus obtaining an electrically connected structure. When the via holes are formed to be fine by a processing means such as laser, an inner surface of the via hole may be minutely grooved and the stepped cone-shaped rough surface may be formed between the inner via hole surface and the conductor layer.
When another via hole is formed above the formed via hole, the upper via hole should be formed to have a larger diameter, compared to the lower hole, with the aim of layer connection. In addition, upon formation of the insulating layer in the via hole by electroless copper plating, short-circuit of the lower conductor circuit layer is caused and inferior goods are produced.
In the case of plugging the via hole with paste, fine or deep via holes may not be completely filled. Also, layers on PCB may be cracked due to gas generated by flux added to the paste.
As for the method of laminating the bump-formed metal foil, the bump should have certain mechanical properties and the used bump is limited in its shape or size. Hence, such bumps, made by exposure during the etching process, are unsuitable for fine pitch and are limited in their use.